Life On Land May Be Much Older Than We Thought

Conventional theories have placed life on land for the last 500 million years, but a new study from a team of American and Australian researchers might push that back to 2.2 billion years.

To support their claim, the scientists presented evidence in the form of tiny newly discovered fossils the size of a match head called Diskagma buttonii that were discovered in ancient soil samples.

“They certainly were not plants or animals, but something rather more simple,” said co-author Gregory Retallack, co-director of paleontological collections at the University of Oregon’s Museum of Natural and Cultural History.

The fossils are small, vase-like structures with a cup on one end and a basal tube on the other end. Retallack says these ancient organisms are comparable to a modern soil organism called Geosiphon, a fungus containing a cavity filled with symbiotic cyanobacteria.

“There is independent evidence for cyanobacteria, but not fungi, of the same geological age, and these new fossils set a new and earlier benchmark for the greening of the land,” he said. “This gains added significance because fossil soils hosting the fossils have long been taken as evidence for a marked rise in the amount of oxygen in the atmosphere at about 2.4 billion to 2.2 billion years ago, widely called the Great Oxidation Event.”

That event, which occurred around 2.4 billion years ago, boosted atmospheric oxygen to around 5 percent – still a far cry from today’s level of 21 percent.

Identifying the fossils was difficult, according to Retallack, because Diskagma are too large to be completely visible on microscopic slides and the fossils were contained in rock too dark to see through. Using a powerful particle accelerator at the Lawrence Berkeley National Laboratory in California, the scientists were able to create a three-dimensional replica of the organism’s odd shape.

“At last we have an idea of what life on land looked like in the Precambrian,” Retallack said. “Perhaps with this search image in mind, we can find more and different kinds of fossils in ancient soils.”

In their conclusion, the scientists said their newly describedfossil is similar to Thucomyces lichenoides, a fossil dating back 2.8 billion years ago and also found in South Africa. However, the interior structure and trace elements of Thucomyces is markedly different.

The research team also noted similarities between Diskagma and three living organisms: the slime mold Leocarpus fragilis, the lichen Cladonia ecmocyna, and the fungus Geosiphon pyriformis.

The new fossil could be the oldest known organism to contain a nucleus and other complex cell structures, the author said. Known as eukaryotes, these simple organisms have a true nucleus, unlike bacteria and algae, and have been dated back as far back as 2.1 billion years. Previous studies have identified biomarkers suggesting that eukaryotes could have arisen even earlier.

“Researchers at the UO are collaborating with scientists from around the world to create new knowledge with far-reaching applications,” said Kimberly Andrews Espy, UO vice president for research and innovation, and dean of the graduate school. “This research by Dr. Retallack and his team opens new doors of inquiry about the origins of ancient life on Earth.”